Many types of batteries, such as lead-acid, nickel-cadmium, and lithium-ion, can be recharged to replenish their charge and thus be used again to power a device such as an MP3 player, an electric motor for a golf cart, or a starter motor for an internal combustion engine. An advantage to using a rechargeable battery to power a device is that one does not have to purchase many single-use batteries to power the device.
The process for recharging a battery involves applying a current to the battery that is opposite in polarity to the discharge current generated by the battery. The applied current reverses the battery's chemical process that occurs in the discharge cycle, and causes material to be deposited on and/or removed from one or more of the battery's electrodes. Some recharge processes provide the depleted battery a constant current at a voltage that is slightly higher than the standing voltage of the battery when it is fully charged. A problem with this process is that the current does not decrease as the battery nears its full charge capacity. Thus, the battery receives more current than the chemical process can consume when the battery nears it charge capacity. The excess current can damage the battery by:                1) Converting a portion of its electrolyte into gas which is vented from the battery,        2) Improperly removing material from or depositing material to an electrode of the battery, or        3) Excessively heating the battery.        
Another recharge process provides the depleted battery a current at a constant voltage that is slightly higher than the fully recharged capacity of the battery. Thus, as the depleted battery is recharged, the voltage difference between the charging source and the battery decreases, causing the current delivered to the battery to decrease. One problem with this process is that it takes significantly longer for the depleted battery to reach its full charge capacity at the end of the recharge cycle. Another problem is that the battery can suffer the same damaging effects of the constant current recharge process during the beginning of a constant voltage recharge cycle because there is an excessive current caused by an initially high difference in voltage between the charging source and the battery at the beginning of the recharge cycle.
A problem common to both the constant current and constant voltage charging methods is the inability of the battery to completely reverse all of the battery chemistry to the original condition it had before it was discharged. In other words, with each discharge/recharge cycle there exists a portion of the battery's chemistry that is not converted back to the charged condition. This results in successive degradation of the battery with each discharge/recharge cycle until the battery's capacity is lowered beyond a state of practical use and must be replaced.